1. Field of the Invention
The present invention relates to a composition for forming an injectable hydrogel.
2. Description of the Related Art
Hydrogels are three-dimensional hydrophilic polymeric materials and possess numerous advantages such as biocompatibility, the ability to absorb water, and lowinflammatory responses. Hydrogels can also be used as reservoirs for sustained release of drugs or proteins. Particularly, injectable hydrogels are suitable for use in drug/protein delivery and tissue engineering. Injectable hydrogels can be prepared using responses to environmental stimuli such as temperature or pH. For example, such materials are flowable before administration, but once injected, they are rapidly converted into a gel state under physiological conditions. These systems enable mass delivery in a minimally invasive manner, are gelled at desired tissue sites, and minimize the formation of scars, resulting in a reduced risk of infection. Furthermore, bioactive molecules or cells can be incorporated into the gels by simple mixing prior to injection.
Polymers with lower critical solution temperature (LCST) characteristics are soluble below the LCST but become insoluble at or above the LCST, leading to gel formation. Accordingly, they can be utilized as thermosensitive injectable hydrogels. However, the application fields of such polymers are limited, mainly because of very rapid dissolution after injection. The reason for this limitation is that the injection sites of the body are not maintained in a sealed state but are excessively exposed to aqueous environments. There exist studies on the introduction of additional covalent crosslinking bonds, mainly by photopolymerization, in order to improve the stability of the gel state after injection (T. Vermonden, N. E. Fedorovich, D. vanGeemen, J. Alblas, C. F. van Nostrum, W. J. A. Dhert, W. E. Hennink, PhotopolymerizedThermosensitive Hydrogels: Synthesis, Degradation, and Cytocompatibility, Biomacromolecules, 2008, 919-926.; K. J. Jeong, A. Panitch, Interplay between covalent and physical interactions within environment sensitive hydrogels, Biomacromolecules, 2009, 10, 1090-1099).
Pluronics are triblock copolymers of hydrophobic propylene oxide and hydrophilic ethylene oxide and are widely applied as injectable hydrogel systems (K. Mortensen, J. S. Pedersen, Structural study on the Micelle Formation of Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) Triblock Copolymer in Aqueous solution. Macromolecules, 1993, 26, 805-812). Pluronics self-assembled into micelles in aqueous solution and exhibit temperature-responsive sol-gel transition behaviors at or above their critical micelle concentration (CMC) and critical micelle temperature (CMT) values. Gelation of Pluronic proceeds due to micellar packing at or above the CMT and a high concentration of Pluronic, bringing about a dramatic change in the rheological properties of the Pluronic. Based on this thermoreversible gelation, Pluronic F 127 (PF127) has been extensively investigated, especially as an injectable system for local delivery of drug (M. Morishita, J. M. Barichello, K. Takayama, Y. Chiba, S. Tokiwa, T. Nagai, Pluronic F127 gels Incorporating Highly Purified Unsaturated Fatty acids for Buccal delivery of Insulin, Int. J. Pharm., 2001, 212, 289-293). However, similarly to other physical gelation systems, PF127 hydrogels are very rapidly dissolved, which limits their applications. As a result, PF127 hydrogels suffer from very poor in vivo mechanical properties as well as very rapid drug release.
In recent years, biological applications of graphene have received great attention in various applications, for example, biosensors, nanocarriers for drug delivery, and probes for cellular and biological imaging. Graphene has a structure in which a monolayer of sp2-hybridized carbon atom is arranged in a two-dimensional honeycomb crystal lattice. Graphene is considered as a planar aromatic macromolecule due to its π-conjugated layer structure. This planar structure gives a very high degree of surface area and imparts the ability to react with various materials, including metals, drugs, biomolecules and cells. However, graphene necessitates surfactants or surface modification for biological applications because it is highly hydrophobic and is not readily dispersible in water. In contrast, graphene oxide (GO) is an oxidized derivative of graphene and is a compound consisting of carbon, oxygen, and hydrogen in various ratios. GO has hydrophilic groups such as carboxyl, hydroxyl and epoxy groups, which impart water dispersibility. Although GO is relatively hydrophilic compared to graphene, the biocompatibility and toxicity of GO have not been elucidated to date. Most studies have reported that GO does not exhibit in vitro and in vivo toxicity, but some researchers have reported the toxicity of GO (K. H. Liao, Y. S. Lin, C. W. Macosko, C. L. Haynes, Cytotoxicity of graphene oxide and graphene in human erythrocytes and skin fibroblasts, ACS Appl. Mater. Interfaces, 2011, 3, 2607-2615). Under these circumstances, GO sheets modified with nontoxic polymers have been developed and investigated for bioapplications.
The present inventors also reported a stimulus-sensitive injectable hydrogel system composed of self-assembled graphene oxide nanosheets mediated by physical crosslinking of Pluronic in a low concentration solution of the Pluronic (Korean Patent Publication No. 10-2013-0115459). This hydrogel was reported to undergo a sol-gel transition in response to various stimuli and to form a stable gel without causing any noticeable immune reactions after in vivo subcutaneous injection. The present inventors also reported the fact that Pluronic-coated nano-graphene oxide functions as a delivery vehicle for photothermal materials and photosensitizers, creating synergistic effects in phototherapy (A. Sahu, W. I. Choi, J. H. Lee, G. Tae, Graphene oxide mediated delivery of methylene blue for combined photodynamic and photothermal therapy, Biomaterials, 2013, 34, 6239-6248). The two documents show a strong affinity between graphene oxide and Pluronic and reveal the fact that the GO-Pluronic composite systems have acceptable biocompatibility.
Several research groups have reported the fact that the introduction of GO considerably increased the mechanical and thermal properties of host polymers (J. Shen, B. Yan, T. Li, Y. Long, N. Li, M. Ye, Mechanical, thermal and swelling properties of poly(acrylic acid)-graphene oxide composite hydrogels, Soft Matter, 2012, 8, 1831-1836). Li et al. prepared pH- and temperature-responsive hydrogels using linear sodium alginate and GO-crosslinked poly(N-isopropylacrylamide)(PNIPAM) and reported that the GO composite hydrogel shave much stronger mechanical properties than GO-free hydrogels (Z. Li, J. Shen, H. Ma, X. Lu, M. Shi, N. Li, M. Ye, Preparation and characterization of pH- and temperature-responsive hydrogels with surface-functionalized graphene oxide as the crosslinker, Soft Matter, 2012, 8, 3139-3145). Zhang et al. reported the fact that GO as a nanofiller was introduced into polyvinyl alcohol (PVA) as a matrix to prepare composite hydrogels with improved tensile strength, elongation at break and compressive strength (L. Zhang, Z. Wang, C. Xu, Y. Li, J. Gao, W. Wang, Y. Liu, High strength graphene oxide/polyvinyl alcohol composite hydrogels, J. Mater. Chem., 2011, 21, 10399-10406). However, to the best of our knowledge, there has been no report on the use of GO for the purpose of improving the in vivo stability of physical gel systems.